Although the Leader is fighting Captain America in the MCU, he has a very long history as the Hulk's primary enemy.

Brave New World features a new look at Leader, the secret villain pulling the strings in the Marvel film. Tim Blake Nelson makes his return to the Marvel Cinematic Universe in Captain America: Brave New World,

The second film in the MCU has rarely factored into the universe as a whole, beyond William Hurt’s Thaddeus “Thunderbolt” Ross making recurring appearances in later films. That changes with Captain America: Brave New World, which will see Liv Tyler and Tim Blake Nelson reprise their roles as Betty Ross and Samuel Sterns/Leader, respectively.